Hi aWc,
I'm not experiencing that each time - but we can definitely get some results like you describe.
I do believe there's no getting around getting in under the hood with EQ and other things (like some artful painting in of a volume dip for certain ranges by clicking "Edit" in the Morph panel to select the instrument to attenuate, then click up top on "Note edit" and choose "Volume" - then take time to lower a nice curve down, or up, in keyboard ranges where there's any surplus heft, or lack. Don't be afraid to paint a scratchy looking curve, the "Smooth" button is our friend and can turn a ragged line we draw into a smooth one).
Here are some more ideas from thinking about what I'd do if I felt I ended up with too much low-mids (I know this range only all too well esp. with my early days with consumer equipment... hello low-mids, my old friend, you've come to ruin my song again.. etc. )
I like to flatten the morph when the first run of mixing the multiple instruments together is balanced (to the point I think "OK, I have all the character from each now"). Then I might be better able to address any imbalances which arose from mixing these beasts together.
Maybe what you describe can be fairly painlessly ironed out by putting an EQ3 in, to create a dip (or pre-tsunami tide) in any excited EQ regions.
Mostly, I found by mixing the instruments' levels (even down to single digits on those mix sliders) you can avoid making some saturation in a region - and I like to click Edit for each piano and remove a little bass or mids (whatever is prominent) to help with the mix.
That's pretty much an outline of how I go at it - definitely beginning by considering what I want from the mixed instruments, but after the flatten (and of course saving that as a preset, usually with version naming convention like '2piano-morph-Ab' or other), most decorative or curative editing happens after the morph, in my flow.
The interesting thing to me, is each instrument is a different beast, and adding and mixing them is all about mixing the physics from these (including what mics are doing etc.). Thinking of something analogous like Photoshop layers, each instrument is not an opaque layer - instead, more like the artfully transparent ones which add up to a certain look. I think they knocked it out of the park but yes, I do think each morph can have as much, or more essential tweaks to EQ and other things to make a stable final flat FXP.
Hope that helps you out aWc. In coming updates the morphing algorithms will likely be incrementally given new bounds - what you bring up is likely a really good marker to consider if Modartt team is looking at first order things to smooth out.
It's a strange thing audio, often range of lower mids to bass can have the most ear-busting impaction - so if any area of EQ will be compounding when mixing tracks/pianos to negative effect, it's often a variant range around 500kHz (human ears like a little dip in that range and taper back up around 350 maybe as a first thing to try (maybe save an EQ3 "low-mid dip" preset so you can load in that as a starting point) - it might be my best EQ tip.. so easy, a dip around 500kHz.. not new, not my idea but it's something not thought of outside audio-land).
I like flat monitors for this - with sweet or monitors like you have, I always tend to push too much bass (mixes, presets). I know you like your monitors (and I'm not negative about them) but I do know, I have tons of old tapes and mixes which all exhibit too much "boom" and muddy thumping low-mids simply because in those spaces we didn't have proper flat monitoring or large enough speakers to hear this well enough to judge it from input to output.. for example, when recording "push up the bass it's so thin".. on mixing out "push up a little more low-mids, it's vacant".. and finally when taking any of this to a nice stereo "Oh dear, booming, muffled, ear-hurting bass-heavy mangled sounds, like the studio was inside a walrus' belly"
Pretty un-edited all the above, but time is short today - hope all is well with you and hope these ideas go some way to helping wrangle some nice and creamy results from your morphing aWc. (will probably be back online in around 12hrs). Cheers!
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors